1. Field of the Invention
The present invention relates to a frequency modulation-type transmitting apparatus that can automatically adjust a frequency modulation factor.
2. Description of the Related Art
Frequency modulation (FM) can be applied to stereo audio signals. A transmitting apparatus is generally configured to output electromagnetic waves carrying a modulated signal via an antenna.
FIG. 11 is a block diagram showing a fundamental arrangement of a conventional frequency modulation type transmitting apparatus 100 applicable to stereo audio signals. The transmitting apparatus 100 includes a front-end processing circuit 10, a frequency modulation circuit 12, an amplifying circuit 14, and an antenna 16.
The front-end processing circuit 10 includes two preamps 10a, two limiters 10b, two low-pass filters 10c, and a single mixer 10d. The front-end processing circuit 10 receives right and left audio signals from an external player or an audio LSI. The right audio signal is processed by one set of the preamp 10a, the limiter 10b, and the low-pass filter 10c dedicated for the right audio signal. The left audio signal is processed by the other set of the preamp 10a, the limiter 10b, and the low-pass filter 10c dedicated for the left audio signal. The mixer 10d mixes the processed right and left audio signals and outputs a composite signal Sc.
The frequency modulation circuit 12 includes an oscillation circuit 12a. The oscillation circuit 12a shown in FIG. 11 is a clapp oscillator including a varactor diode D whose capacitance is varied in accordance with the composite signal Sc supplied from the front-end processing circuit 10. The frequency modulation circuit 12 outputs a frequency modulated signal SFM having a frequency corresponding to the amplitude (voltage value) of the composite signal Sc. More specifically, the frequency modulation circuit 12 operates as a voltage controlled oscillator (VCO) that modulates a change occurring in the amplitude (voltage value) of the composite signal Sc into a change in the frequency of the frequency modulated signal SFM.
The amplifying circuit 14 amplifies the frequency modulated signal SFM produced from the frequency modulation circuit 12 and transmits the amplified signal via the antenna 16.
When the frequency modulated signal SFM is demodulated, a receiving circuit including a frequency modulation detector can be used. The frequency modulation detector receives the frequency modulated signal SFM and demodulates the change occurring in the frequency of the frequency modulated signal SFM into a change in the voltage of an output signal, and outputs a demodulated signal.
When the frequency modulation detector inputs a carrier, i.e., a signal being not yet frequency modulated, the non-modulated noise depending on a modulation factor B is produced from the frequency modulation detector. The non-modulated noise does not depend on a frequency modulation factor A of the transmitting apparatus 100. More specifically, the receiving apparatus produces an output signal having an S/N ratio equal to a ratio of the modulation factor B to the modulation factor A.
Accordingly, to realize an excellent S/N ratio, it is desirable to increase the frequency modulation factor A of the transmitting apparatus 100. On the other hand, the modulation factor B allowable by the frequency modulation detector in the receiving apparatus is limited.
In view of the foregoing, it is desirable to set an appropriate value for the frequency modulation factor A of the transmitting apparatus 100 with reference to the modulation factor B allowable in the frequency modulation detector. In general, the modulation factor can be expressed as a ratio of the carrier to the signal.
The transmitting apparatus 100 shown in FIG. 11 does not include any arrangement capable of controlling the frequency modulation factor A. It is therefore necessary to control the amplitude of a signal applied to the varactor diode involved in the frequency modulation circuit 12. The following are practical methods presently available for controlling the amplitude of a signal applied to the varactor diode.
(1) A user can manually adjust the level of a signal input to the transmitting apparatus 100. For example, a user can manually adjust the volume of an external player or an audio LSI. Adjusting the level of the signal input to the transmitting apparatus 100 can appropriately control the frequency modulation factor of the transmitting apparatus 100. It is also possible to store, in a register, the signal level when the user manually adjusted the external player or the audio LSI, and provide a circuit capable of automatically equalizing the signal level to a registered value.
(2) As shown in FIG. 12, the front-end processing circuit 10 can include two electronic voltage controllers 10e each intervening between the preamp 10a and the limiter 10b. The electronic voltage controller 10e can adjust its output depending on a setting value of a register 10f. A user can manually change the setting value of the register 10f to adjust the level of a signal input to the frequency modulation circuit 12, so that the frequency modulation factor A of the transmitting apparatus 100 can be appropriately controlled.
However, the above-described methods (1) and (2) require a user's manual operation (volume control) to adjust the signal level or a control circuit equipped with a memory (e.g., a register) capable of storing the signal level being manually set as an appropriate value.
It is generally difficult and impractical to rely on a user's manual adjustment to constantly maintain the signal level to an appropriate value. Furthermore, when the control circuit including a memory capable of storing a predetermined signal level is provided, the circuit scale of the transmitting apparatus 100 becomes larger and the manufacturing cost will increase.